The present invention is directed to an improved nucleate boiling heat transfer surface. In the preferred embodiment, the surface is formed as a heat transfer tube with water flowing through the inside of the heat transfer tube and refrigerant boiling on the outside of the heat transfer tube. More specifically, the present invention contemplates a heat transfer tube for application in the evaporator of a water chiller.
U.S. Pat. No. 3,881,342 to Thorne shows heat transfer tubing formed with generally circumferentially extending adjacent fin convolutions. The fin convolutions are provided with recesses in the outer edges and the fin convolutions are each bent uniformly towards the adjacent convolution to partly enclose the spaces between adjacent convolutions.
U.S. Pat. No. 3,768,290 to Zatell shows a similar arrangement where the fins are closely adjacent to the next adjacent fin convolution so as to provide small gaps of predetermined and controlled average size.
It is well known that there is an optimum recess size for a given refrigerant at a given heat flux. In an arrangement such as that of the Thorne patent, when the adjacent convolutions are rolled to the point of touching and when the recesses are all of the same size and are sized for maximum heat flux, the recesses will be too large at part load and the cavities beneath the rolled over fin convolutions will become flooded with liquid. This causes heat transfer performance to deteriorate.
It is an object, feature and advantage of the present invention to solve the problems in prior art externally enhanced heat transfer tubes and surfaces.
It is an object, feature and advantage of the present invention to provide a heat transfer surface having notched and rolled fin convolutions where the heat transfer performance is satisfactory at full and part load.
The present invention provides an improved heat transfer surface. The improved heat transfer comprises: a surface covered with fin convolutions. The fin convolutions have fin tips extending from the surface. The fin tips have a first plurality of notches and a second plurality of notches wherein the first notches and the second notches are of different sizes.
The present invention also provides a heat transfer tube for use in an evaporator tube or tube bundle. The tube includes an annular wall or base member having an inner surface, an outer surface and an elongate access. The tube has an inner rib on the inner surface of the annular wall, and a plurality of axially spaced fin convolutions on the outer surface of the annular wall. Sectors having precisely sized and designed indentations are located at specific intervals along an extreme outer edge of the axial spaced fin convolutions. Each of the precisely sized and designed indentations on an individual fin has a different design depth or size than an immediately adjacent indentation. Each fin convolution is bent over so that a tip of each fin convolution is brought into contact or overlapped contact to a side of an adjacent fin convolution and defines an elongated circumferential tunnel or enclosed cavity. Each bent over fin convolution is of curvilinear cross-section over substantially its entire length starting from a skewed plane normal to an elongate tube axis. Each of the indentations on the bent over fin convolution forming precisely, different shaped and sized pore openings communicating with the tunnel. The pore openings allowing a media or refrigerant to continuously fill and flow inside the tunnels whereby the heat exchanged through the inner surface, the base member and the fin convolutions will promote and sustain a nucleate boiling process in the media at a maximum efficiency over a wide range of heat fluxes.
The present invention further provides a method of making a heat exchanger tube. The method comprises the steps of: providing a tubular blank having a generally circular cross section of a predetermined outer diameter; forming an extended heat transfer surface by extruding a helical fin up from the outer surface of the tubular blank; applying lateral force to one side of the helix of the helical fin to cause the helical fin to bend over; and notching the side of the helix of the helical fin to form pores of at least two different sizes.
The present invention still further provides a method of making a heat exchanger tube. The method comprises the steps of: providing a tubular blank having a generally circular cross section of a predetermined outer diameter; forming an extended heat transfer surface by extruding a helical fin up from the outer surface of the tubular blank; notching the side of the helix of the helical fin to form pores of at least two different sizes; and applying lateral force to one side of the helix of the helical fin to cause the helical fin to bend over.
The present invention yet further provides a method of providing a heat exchanger tube having continuous helical fins thereon with a plurality of first and second size cavities in the periphery of the fin. The method comprises the steps of: deforming the periphery of the fins to less than the full depth thereof to form a first size cavity thereon; deforming the periphery of the fins to less than the full depth thereof to form a second sized cavity thereon wherein the second size cavity is of different size than the first size cavity; and rolling the tips of the helical fins to touch the side of the adjacent helical fin.